1. Field of the Invention
The present invention relates generally to a method and device for providing a programmed active exercise treatment for increasing the amount, strength and proper anatomical distribution of skeletal tissue in a patient suffering from a bone disorder.
2. Description of the Prior Art
The present invention relates to a number of disorders of skeletal tissue in which an active exercise treatment may be employed. These disorders include situations involving both acute and chronic fractures of bones, replacement of joints with artificial prostheses, leg-lengthening procedures, and generalized or diffuse osteoporosis.
When a bone is broken, acute fracture healing is triggered by a so-called "injury potential" which can be measured across two sides of the fracture. As early healing progresses, governed in part by bioelectrical mechanisms, the ends of the bone become joined by a tissue known as "bridging callus," which serves to anchor the ends of the fractured bone to one another. With time, this tissue is remodeled from a weak, woven (fetal), unstructured bone, to strong, well-organized, highly structured bone tissue. This maturation phase of the fracture repair process may be enhanced by applying compressive loads to the bone, directed along its axis, and of appropriate amplitude and rate of loading. This phase is also mediated by bioelectric processes, as mechanical energy is transduced by the piezoelectric and electrokinetic properties of bone to a modification of the activity of the bone cells in selected ways and at selected sites (discussed below under Scientific Studies). This stress working process serves to hasten maturation of the newly formed, unstructured, repair-bone, and consequently reduces the amount of time a limb needs to be externally immobilized (e.g., to be in a cast or a frame). Furthermore, cast immobilization and fracture repair are often accompanied by a depletion of bone mass (localized disuse osteoporosis) in structures at a considerable distance from the fracture itself. In weight bearing bones, rehabilitation often is retarded by stress pain in response to the bone loss which accompanies casting. Internal fixation with nails or plates, also, results in disuse osteoporosis as the result of stress-relief, the repair process itself, and motor disability.
It is well recognized by orthopaedic surgeons and other physicians that early functional use of a broken extremity is desirable to speed a patient's rehabilitation. Few doctors or patients, however, have recognized that the benefits of function (e.g. weight-bearing) derive, mainly, from brief intervals of controlled axial compression loading at critically rapid rates (i.e., impacting). In fact, most patients, after fracture, are unable to load with appropriate impact unless taught specific methods with effective monitoring methods to achieve this end. Furthermore, loading patterns which do not produce axial impact compression may introduce mechanically-deleterious torque, shear, or bending moments at rates too slow to improve the function of bone cells.
Thus it is desirable to provide a means for individuals with fractures to achieve appropriate compressive loading of their fractured bones to accelerate the maturation (strengthening) process. The loading patterns of these compressive forces should be controlled so that the stimulus for remodeling is below that which would produce acute or fatigue failure of the structurally evolving new bone.
About 5% of long bone fractures fail to heal in the normal tissue and fashion. In these cases the long bone fractures fail to unite and proceed to "delayed union" or "non-union." These conditions are characterized by a persistence of soft tissue opposite and within the fracture gap. In order to institute the final phases of repair, usually months to years after the original injury was sustained, it is necessary to initiate calcification and vascularization of these soft tissues. A commonly used method to achieve these ends is the use of selected pulsed electromagnetic fields delivered through a coil(s) attached to the cast over the old fracture site. Once the repair process is re-instituted, both the surgeon and the patient are desirous of reducing the total time required in cast before unrestricted function can begin. Rapid maturation of the bridging, unstructured new bone, without overloading, is a sine qua non for early rehabilitation. The principles of controlled, active, axial compression exercise to achieve these ends have been enunciated and clinically used successfully for the past ten years, but without an effective device to guide the patient in the loading program.
Osteoporosis is a chronic disorder which usually, but not exclusively, afflicts older women. Others who may be affected by this disorder include those who are confined to bed and even astronauts who are in a weightless environment. Osteoporosis is characterized by a decrease in the density of mineralized bone mass which makes the affected bones more fragile and therefore more susceptible to breakage.
Osteoporosis is frequently a debilitating problem. The injuries which result from osteoporosis often require extended hospitalization, and sometimes involve costly and painful surgery (e.g. total hip joint replacement). Health care costs for this condition approach ten billion dollars per annum in the United States alone. In addition, osteoporosis severely diminishes the vitality and mobility of those who suffer from this disease.
The general population also feels the effects of this disorder. Individuals who are afflicted with osteoporosis must depend upon relatives and others for care, and the health care and hospital costs are borne by everyone.
Osteoporosis occurs when the destruction of bone occurs at a rate faster than that with which new bone formed. The balance between destruction and formation is governed by hormones, calcium intake, vitamin D and related compounds, weight, smoking, alcohol consumption, exercise and other factors.
Much effort in the medical community has been focused on slowing or reversing bone loss through administering estrogens, calcitonin, calcium, fluorides, and thiazides, and recommending exercise. None of these modalities has been entirely successful in restoring bone mass to a severely depleted skeletal system.
Thus, it is desirable to find new methods for treating osteoporosis. A promising avenue is based upon a physiologic principle known as Wolff's law, which states that bone adapts its internal structure in response to the forces which act upon it. In other words, bone will remodel itself so that it is optimally structured to bear the applied stress.
Research has shown that Wolff's law is enacted, in part, through bioelectric processes. Because bone is piezoelectric and electrokinetic, it generates an electrical signal in response to mechanical forces. This internally-generated electrical signal then has a positive effect on bone formation. The principles of axial impact exercise just noted for fracture care apply equally well for osteoporosis. Not only can they prevent bone loss but they can restore bone mass and strength, once lost. The key to their success in this pathologic entity, again, rests on achieving a critically rapid skeletal loading rate to activate bone forming cells. For individuals with low bone mass, the amount of loading must be consonant with the amount of residual bone and it is increased as the mass increases in response to appropriately controlled active exercise.
Joint replacement surgery now involves two major types of bonding between the endoprosthesis(es) and bone. One makes use of a filler material (glue), such as methyl methacrylate. The second, newer method relies on the ability of bone to grow into a porous surface of the implant (metal, plastic, or composite), thereby locking the device in place. Biologically, the postsurgical response is similar to fracture healing, with an initial deposition of woven (fetal), unstructured bone at the interface between host bone and the implant and within its porous interstices. The rate of rehabilitation following joint replacement in the lower extremities is determined by the rate at which interfacial new bone can be stress-worked (remodeled) without a shearing failure. Excessive, early loading can convert new bone into fibrous tissue, producing a post-surgical failure. It is important, if not imperative, therefore, to control the amount of applied load and to keep its rate of increase consistent with the ability of interfacial bone to mature without a materials or cellular failure.
In order to equalize significant leg length inequality in adults, a mid-shaft (diaphyseal osteotomy often is performed after the application of a distractable external fixator. When the early repair of this iatrogenic fracture is in progress, at about 3-4 weeks post-operatively, daily controlled distraction is begun and continued until limb length equality is achieved or approached. Post-lengthening, the return of sufficient strength to the operated limb to permit unrestricted function is determined by loading patterns. Again, controlled, active, axial compressive impact exercise can be a useful adjunct to increase the rate of maturation without a material failure in the repairing segment.
The interactions between bone structure and mechanical forces has been studied scientifically. One of the first and most complete investigations into the effects of mechanical loading on bone tissues was reported by Cochran et al. in "Electromechanical Characteristics of Bone Under Physiologic Moisture Conditions," (Clinical Orthopaedics, 58:249-70, 1968). In that publication, it was shown that electrical potentials were developed in bone in response to mechanical stresses, both with in vivo and in vitro studies. This work contributed to the successful use of electromagnetic stimulation to modify bone tissue, as reported by Bassett et al. in "Augmentation of Bone Repair by Inductively Coupled Electromagnetic Fields," (Science, 184:575-77, May 1974) and Bassett et al. "A Non-Operative Salvage of Surgically Resistant Pseudarthroses and Non-Unions by Pulsing Electromaqnetic Fields, A Preliminary Report," (Clinical
Orthopaedics, 124:128-43, 1977). The importance of bioelectric phenomena in osteoporosis has been reported in part by Bassett et al. in "Prevention of Disuse Osteoporosis in the Rat by Means of Pulsing Electromagnetic Fields" (Brighton, et al., Electrical Properties of Bone and Cartilage: Experimental Effects and Clinical Applications, 1979), and by Cruess et al. in "The Effect of Pulsing Electromagnetic Fields on Bone Metabolism in an Experimental Model of Disuse Osteoporosis" (Clinical Orthopaedics, 173:245, 1983).
In the paper by Cochran, et al. (above), it was demonstrated that the mechanical loading of bone needed to occur at a particular rate in order to generate maximal voltages. To this end, patients have been treated with axial compression exercise, at prescribed rates of loading, as reported by C.A.L. Bassett, "Effect of force on skeletal tissues", (Physiological Basis of Rehabilitation Medicine, Downey and Darling eds., 1st ed., W.B. Saunders Co., 1971, pp. 312-314). In these exercises, patients used a fish scale to approximate the maximum impact of their compression exercise, but they had no way to quantify the rate at which the impact took place.
Other research into mechanical methods to control bone loss have been reported For example, the National Aeronautics and Space Administration funded a project to study the use of impact loading on individuals' heels to stimulate bone formation. Reference to this work was made in an abstract printed in the USPHS Professional Association, 11th Annual Meeting (May 1976) proceedings, and entitled "Modification of Negative Calcium Balance and Bone Mineral Loss During Bed Rest." The abstract reported that impact loading, which was limited to a maximum of 25 pounds, could slow down the loss of calcium.
Rubin and Lanyon have also investigated the relationship between mechanical forces and bone formation, and have suggested that periodic strain rates and cyclic patterns generate a maximal osteogenic response in avian bones. In "Regulation of Bone Formation by Applied Dynamic Loads", (Journal of Bone and Joint Surgery, 66-A(3): pp. 397-402, March 1984), cyclic loading at 0.5 Hz caused bone formation to be augmented. In "Regulation of Bone Mass by Mechanical Strain Magnitude," (Calcified Tissue International, 37:411-417, 1985), it was shown that a dose relationship exists between peak strain applied and change in bone tissue mass.
The challenge of utilizing these facts is to translate this general laboratory information into clinically effective devices and methods for treating the bone disorders discussed above.
It is therefore an object of the present invention to devise a treatment method and device for selected bone repair situations which are both safe and effective.
It is a further object of the present invention to employ the concept of a critical loading factor in the treatment method and device.
Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.